1. Field of the Invention
The present invention relates to an operation-control lever unit for regulating power of an engine of an engine-powered working machine to control operation of a working tool of the engine-powered working machine. The engine-powered working machine may include a carrying bush cutter, a chemical sprayer, a vacuum dust collector and so on.
2. Description of the Related Art
A carrying bush cutter having a circular cutter driven for rotation by an engine carried on the back of an operator is known from Japanese Utility Model Publication No. (SHO) 63-14035. The circular cutter is attached to one end of a hand-operating rod. The rod is provided with an operation handle near the other end thereof. In use of the known bush cutter, the operator swings the rod up and down and right and left while gripping the operation handle. With this swinging operation, bushes are cut or removed by the rotating circular cutter.
In order to control the rotational speed of the cutter, output power of the engine is regulated by a throttle lever provided on a grip portion of the operation handle. However, because the operator is forced to continue gripping of the throttle lever and the operation handle throughout the bush-removing work, a heavy work load is put on the operator.
According to somewhat successful prior improvements, a lock mechanism is associated with a throttle lever to temporarily lock the throttle lever in a desired position, so that the work load on the operator can be reduced. Typical examples of the prior improvements are disclosed in Japanese Utility Model Publication Nos. (SHO) 53-42661 and (SHO) 55-21536, Japanese Utility Model Laid-open Publication No. (SHO) 60-41539, and Japanese Patent Laid-open Publication No. (HEI) 8-303263.
According to the disclosed operation lever units, the throttle lever is displaced to a predetermined operating position, then locked in this operating position by activating the lock mechanism. The lock mechanism is released at need.
More specifically, the operation lever unit disclosed in Japanese Utility Model Publication Nos. (SHO) 55-21536 includes a throttle adjustment lever and a throttle release lever provided side by side on the operating handle. However, because these levers have different axes of pivotal movement, the operation lever unit is complicated in construction, requires an increased number of structural components, and is uneasy to manipulate. Additionally, due to wear and deformation occurring during a long period of use of the engine-powered working machine, a frictional force acting on the throttle lever to lock the same lever in position against pivotal movement tends to decrease and eventually allows the throttle lever to move from the locked position. The throttle lever may sometimes return to its original position when the operation lever unit is subjected to vibration. The operation lever unit has no means to adjust the frictional force acting on the throttle lever.
In the operation lever unit shown in Japanese Utility Model Laid-open Publication No. (SHO) 60-41539, a throttle lever and a lock lever are mounted on the same pivot pin. However, due to a strong spring force acting directly on respective pivoted portions of the levers, a great muscular effort or force is required to turn each lever. Thus, the manipulability of the operating lever unit is relatively low. If the spring force is weakened, the lock lever will fail to lock the throttle lever at a desired position with sufficient reliability.
The operation lever unit disclosed in Japanese Utility Model Publication No. (SHO) 53-42661 includes a locking pawl pivotally mounted between the pivot axis of a throttle lever and the pivot axis of a lock lever for operatively interlocking the two levers. Because of the presence of the locking pawl, the operating lever unit is complicated in construction and requires an increased number of structural components.
In the operation lever unit shown in Japanese Patent Laid-open Publication No. (HEI) 8-303263, a throttle lever and a lock lever are pivotally mounted on the same pivot pin. However, because the lock lever is normally held stationary against pivotal movement using the force of a spring only, this operation lever unit has the same problem as the operation lever unit disclosed in Japanese Utility Model Laid-open Publication No. (SHO) 60-41539 previously described.